Connecting Multiple PTC Sensors to One Drive Limits and Best

Problem Overview: Two Motors, One Drive, One PTC Input

A common field question is whether a single variable frequency drive (VFD) can monitor the winding temperature of two parallel-fed motors through one PTC thermistor input. The typical scenario: a SINAMICS G120C (or comparable compact inverter) feeds two motors on a common shaft assembly, a dual-pump skid, or a coupled fan/pump pair, and the cabinet designer wants to avoid the cost of a second thermistor relay.

The technical answer is that series-connecting two PTC thermistors to one drive input is electrically possible, but the consequences are not benign: the trip resistance no longer matches the DIN 44081 characteristic, the absolute trip temperature of the chain shifts downward, and the inverter can no longer identify which motor actually overheated. For safety-critical applications (HVAC smoke extract, hazardous area, elevator hoisting) the correct topology is one thermistor evaluation channel per motor. This reference quantifies the problem and provides the decision matrix, parameter set, and wiring rules to apply in the field.

PTC Thermistor Fundamentals: DIN 44081 / DIN 44082

Positive Temperature Coefficient (PTC) thermistors used for motor protection are defined by DIN 44081 (single thermistor) and DIN 44082 (triple thermistor for three-phase windings). They are not linear RTDs - their resistance remains low across the operating range and switches sharply at the rated trip temperature (NAT, Nominal Answering Temperature).

Standard NAT ratings for motor PTCs are 90, 100, 110, 120, 130, 140, 150, 155, 160, 170, 180 °C, and the device is selected by the motor insulation class (Class F → 150 °C, Class H → 170 °C NAT is typical).

The protective relay - whether a stand-alone thermistor relay (Phoenix Contact, ABB CM-MSS, Eaton EMT6) or the integrated PTC input of a VFD - works by sourcing a small measuring current through the sensor and comparing the voltage drop against fixed thresholds. The drive's internal PTC evaluation is therefore functionally identical to a relay; what differs is the trip resistance threshold and the action taken on trip.

Drive-Integrated PTC Evaluation: SINAMICS G120C

The SINAMICS G120C Operating Instructions describe a single PTC/PT1000 input on the Control Unit terminal block. On a CU240E-2 the sensor is wired to terminals T1 and T2 (a 2-wire connection, no polarity). Sensor type selection is made via parameter:

For PTC selection (p0601 = 1), the Control Unit applies the DIN 44081 envelope:

Trip Threshold Comparison: Drive vs Thermistor Relay

The 2000 Ω vs 3500 Ω gap is not a defect - it reflects the difference between an integrated safety function (the drive must guarantee protection within its own fault envelope) and a relay designed to faithfully implement the DIN 44081 curve so that the motor reaches its NAT before the relay commits to a trip. Both are valid; the choice depends on whether you want the drive to guarantee protection or respect the motor NAT.

Series Connection: Resistance Math and Trip Shift

When two identical PTC thermistors are wired in series to a single input, the resistances add at every temperature point. The drive still sees the same nominal 200 Ω cold, 1650 Ω at NAT, and > 4000 Ω beyond NAT - but those values now correspond to different physical temperatures on the windings.

Worked Example: Two Identical 110 °C NAT PTCs in Series

Below NAT the series resistance is roughly double the single-PTC resistance. At the rated trip temperature of 110 °C (single PTC), the resistance is at least 1650 Ω; the second PTC also contributes ~1650 Ω, so the combined loop is at least 3300 Ω - already past the drive's 2000 Ω trip threshold.

The drive therefore trips when each motor is only at ~100 °C, roughly 10 K below the PTC's rated NAT of 110 °C. The absolute protection limit has shifted by approximately 10 K, equivalent to a derating of one full insulation class for an F-class motor (155 °C).

Diagnostic Loss: Motor Identification

A VFD PTC input is a single-channel evaluator. When it trips, the fault buffer stores F07911 "Motor overtemperature" with no indication of which motor caused it. If both motors are inside the same enclosure and fed from one contactor, the maintenance crew must:

1. Megger or resistance-check both PTC chains manually
2. Compare with motor thermal history (ambient, load, current trace)
3. Replace the suspect motor before re-energising

On a dual-pump skid, a single shared PTC therefore hides which pump failed. In process plants this is unacceptable because the cause (seized bearing, blocked strainer, dry run) cannot be diagnosed without further disassembly. For redundant pumps a separate thermistor relay per pump, plus a selector switch to display either on the HMI, is the conventional engineering practice.

Dedicated Thermistor Relay Protection

A stand-alone PTC relay - for example the ABB CM-MSS, Eaton EMT6-K, Phoenix Contact ETR, or Siemens 3RN20xx - is a single-function protective device that:

• Provides one isolated PTC input per motor
• Drives a single changeover contact (typically wired to the drive's enable input, the contactor, or a safety relay)
• Implements the full DIN 44081 envelope (3500 Ω trip, 1700 Ω reset)
• Carries its own fault LED and test button for periodic proof testing

Best practice is to wire the relay's NC contact into the drive's STO / safe stop enable chain or the contactor hold circuit, so that any thermistor trip opens the safety loop and the drive stops regardless of which motor overheated. Each relay is then labelled (PTC1, PTC2) so that the HMI alarm identifies the affected motor.

Allen-Bradley PowerFlex 753 Reference

The same single-input limitation exists on Allen-Bradley drives. Per the PowerFlex 753 PTC input knowledge base answer, the PowerFlex 753 main control board includes a single thermistor input that cannot be paralleled or multiplexed at the I/O level. Rockwell's recommended solutions are:

1. Use an external thermistor relay (e.g., Bulletin 809) per motor, with the relay contact wired to a drive digital input configured for "External Trip" (parameter 7 in PowerFlex 753).
2. Upgrade to the PowerFlex 755 with the 20-750-ATEX option card, which provides dedicated PTC inputs and ATEX-certified motor thermal protection.
3. Where the application permits, replace the thermistor with a UTD, UTH, or KTY-based module in the 11-series I/O slot.

The Rockwell guidance confirms that drive vendors in general do not allow more than one PTC chain per input channel and that the engineering workaround is always an external relay per motor.

Topology Selection Matrix

Wiring and Termination Best Practices

1. Use shielded twisted pair from each motor junction box to the cabinet. Earth the shield at one end only (cabinet ground bar) to avoid ground loops that add series resistance to the measurement.
2. Do not run PTC cables in the same conduit as VFD output cables. Capacitive coupling from the PWM output can inject common-mode voltage into the high-impedance PTC input and corrupt the measurement.
3. Keep the loop resistance of the cable below 10 % of the cold PTC resistance (≤ 25 Ω for a 250 Ω PTC at 25 °C). This ensures wire-break detection still works.
4. Use terminal blocks with screw or push-in cage clamp (Phoenix Contact UT4 or equivalent). Avoid spring-cage terminals that lose tension under vibration, as a high-resistance joint can mimic a hot PTC.
5. Label both ends (T1 / T2 / PTC1 / PTC2) and document in the cable schedule. A common commissioning error is to land PTC2 on T1 and PTC1 on T2, which still works electrically but scrambles the labelling.
6. Verify the cold resistance at commissioning (target: 100 – 750 Ω, drive reads "OK").
7. Inject a test resistor (1.5 kΩ or 4.7 kΩ decade box) in place of the PTC to prove that the drive actually trips at 2000 Ω before handover.

Commissioning and Verification Procedure

1. Power the drive with the motor disconnected. Set p0601 = 1 (PTC) and p0610 = OFF2 (or the desired response).
2. Measure the resistance at T1/T2 with the motor cold. Confirm reading is < 750 Ω and that the drive shows no fault.
3. Open-circuit the sensor (disconnect one wire). Drive should report F07016 "Sensor fault" within 5 s. Reset and reconnect.
4. Short-circuit the sensor (10 Ω jumper across T1/T2). Drive should report F07016 "Sensor short". Reset and reconnect.
5. Substitute a decade box set to 2.2 kΩ across T1/T2. Drive should report F07911 "Motor overtemperature". Reset.
6. Restore the motor PTC chain. Run the motor at rated load for 30 min and confirm r0035 is stable below the p0604 alarm threshold.
7. If two PTCs are series-shared, document the calculated trip-temperature shift (e.g., "ΔT = 10 K at NAT = 110 °C") in the commissioning report and update the motor nameplate label accordingly.
8. For separate thermistor relays, prove each contact opens the drive enable or contactor circuit by simulating a 4.7 kΩ resistance.

Safety and Standards Notes

Motor thermal protection by PTC chain is described in IEC 60947-8 (low-voltage switchgear and controlgear - Part 8: Contactors with motor protection functions) and the integrated drive PTC function is covered in the IEC 61800-5-1 family of adjustable-speed power drive systems standards. ATEX applications require an ATEX-certified thermistor relay with the motor marked "Ex e" and the relay installed per IEC 60079-14. Drive vendors publish their declared sensor thresholds in the operating instructions, but they do not certify the integrated input for ATEX - so the certified relay is mandatory in hazardous areas.

If the motor is variable-speed on a shaft-driven fan (asynchronous induction motor), the drive's I²t thermal model is only an approximation at low speeds because the rotor-driven cooling fan loses effectiveness. The PTC thermistor remains the only direct measurement of the stator winding temperature, which is why the source note that "the inverter can calculate the motor temperature by calculation, but the PTC measures the actual winding temperature" remains the controlling engineering principle.